The project concerns the regulation of ribosome biosynthesis in the lower eukaryote, Neurospora crassa. We studied a duplication strain that has two nucleolar organizer regions instead of one and found that the strain has twice the wild-type number of ribosomal RNA cistrons. Further the number of cistrons decreases upon vegetative propagation. We also studied four heat-sensitive mutants and found that, at the non-permissive temperature, three of them are primarily defective in protein synthesis and secondarily defective in RNA synthesis. Finally we found that when wild type is treated with the protein synthesis inhibitor, cycloheximide, the synthesis and processing of precursor-rRNA is severely inhibited such that very little RNA made in the presence of the antibiotic ends up in mature ribosomal subunits.